One such clutch is described in the specification of French published patent application FR 2 668 234A and the corresponding U.S. Pat. No. 5,119,911. This lock-up clutch consists of a rotatable assembly which has to be balanced dynamically so as to eliminate any out of balance effects in particular, this being known per se.
Such balancing poses certain problems. In this connection it is not possible to pierce the transverse or radial portion of the piston in order to achieve the required balancing, because control chambers are formed on either side of the piston, the control of which is achieved by pressure inversion in these chambers, so that the chambers must be separate from each other.
Consideration can then be given to achieving balancing by forming holes at desired locations in the axially oriented peripheral flange of the piston. However, this is not satisfactory because any operation to form holes is itself a source of pollution: particles, which may for example appear as a result of burrs formed during the drilling operation, are liable to become detached and to contaminate the hydraulic fluid which is contained in the sealed casing in which the lock-up clutch and its associated fluid coupling are enclosed.
It is then also possible to consider a balancing operation using components welded on to the piston itself. This however poses problems of reliability and control, because in order to be certain that the connection has been properly made, it is necessary to use destructive testing.